A flow rate measurement method and apparatus adapted to hydrocarbon well effluents made up of multiphase fluid mixtures comprising water, oil, and gas is known from U.S. Pat. No. 6,405,604. The effluent is passed through a Venturi tube in which the effluent is subjected to a pressure drop, a mean value of the pressure drop is determined over a period of time. A mean value for the density and fractions of the fluid mixture at the constriction of the Venturi tube over said period is determined by means of a gamma ray producing source and an associated detector. A total mass flow rate value is deduced for the period under consideration from the measured mean values of pressure drop and of density. Subsequently, water, oil, and gas flow rates can be calculated from the measured fractions. The apparatus comprises a Venturi tube measuring device and a gamma ray measuring device.
Typically, the gamma ray measuring device is more costly than the Venturi tube measuring device. Further, the implementation of the gamma ray measuring device necessitates some constraints because of the use of a radioisotope. In addition, in case the gamma ray measuring device is removed from the flow rate measurement apparatus, it is not possible to determine water, oil, and gas flow rates anymore.